Method of Equivalent Sources for Removing VRM Responses

Here, we use an equivalent source inversion to remove the VRM response from TEM data collected by a small coincident loop system. The data being inverted are the same as in the forward modeling example. To remove the VRM signal we:

  1. invert the late time data to recover an equivalent source surface layer of cells.
  2. use the recovered model to predict the VRM response at all times
  3. subtract the predicted VRM response from the observed data

Import modules

import SimPEG.VRM as VRM
import numpy as np
from SimPEG import (
    mkvc, Mesh, Maps, DataMisfit, Directives, Optimization, Regularization,
    InvProblem, Inversion
    )
import matplotlib.pyplot as plt
import matplotlib as mpl

Defining the mesh

cs, ncx, ncy, ncz, npad = 2., 35, 35, 20, 5
hx = [(cs, npad, -1.3), (cs, ncx), (cs, npad, 1.3)]
hy = [(cs, npad, -1.3), (cs, ncy), (cs, npad, 1.3)]
hz = [(cs, npad, -1.3), (cs, ncz), (cs, npad, 1.3)]
mesh = Mesh.TensorMesh([hx, hy, hz], 'CCC')

Defining the true model

Create xi model (amalgamated magnetic property). Here the model is made by summing a set of 3D Gaussian distributions. And only active cells have a model value.

topoCells = mesh.gridCC[:, 2] < 0.  # define topography

xyzc = mesh.gridCC[topoCells, :]
c = 2*np.pi*8**2
pc = np.r_[4e-4, 4e-4, 4e-4, 6e-4, 8e-4, 6e-4, 8e-4, 8e-4]
x_0 = np.r_[50., -50., -40., -20., -15., 20., -10., 25.]
y_0 = np.r_[0., 0., 40., 10., -20., 15., 0., 0.]
z_0 = np.r_[0., 0., 0., 0., 0., 0., 0., 0.]
var_x = c*np.r_[3., 3., 3., 1., 3., 0.5, 0.1, 0.1]
var_y = c*np.r_[20., 20., 1., 1., 0.4, 0.5, 0.1, 0.4]
var_z = c*np.r_[1., 1., 1., 1., 1., 1., 1., 1.]

xi_true = np.zeros(np.shape(xyzc[:, 0]))

for ii in range(0, 8):
    xi_true += (
        pc[ii]*np.exp(-(xyzc[:, 0]-x_0[ii])**2/var_x[ii]) *
        np.exp(-(xyzc[:, 1]-y_0[ii])**2/var_y[ii]) *
        np.exp(-(xyzc[:, 2]-z_0[ii])**2/var_z[ii])
    )

xi_true += 1e-5

Survey

Here we must set the transmitter waveform, which defines the off-time decay of the VRM response. Next we define the sources, receivers and time channels for the survey. Our example is similar to an EM-63 survey.

waveform = VRM.WaveformVRM.StepOff()

times = np.logspace(-5, -2, 31)  # Observation times
x, y = np.meshgrid(np.linspace(-30, 30, 21), np.linspace(-30, 30, 21))
z = 0.5*np.ones(x.shape)
loc = np.c_[mkvc(x), mkvc(y), mkvc(z)]  # Src and Rx Locations

srcListVRM = []

for pp in range(0, loc.shape[0]):

    loc_pp = np.reshape(loc[pp, :], (1, 3))
    rxListVRM = [VRM.Rx.Point(loc_pp, times=times, fieldType='dbdt', fieldComp='z')]

    srcListVRM.append(
        VRM.Src.MagDipole(rxListVRM, mkvc(loc[pp, :]), [0., 0., 0.01], waveform)
    )

survey_vrm = VRM.Survey(srcListVRM)

Forward Problem

Here we predict data by solving the forward problem. For the VRM problem, we use a sensitivity refinement strategy for cells # that are proximal to transmitters. This is controlled through the ref_factor and ref_radius properties.

# Defining the forward problem
problem_vrm = VRM.Problem_Linear(
    mesh, indActive=topoCells, ref_factor=3, ref_radius=[1.25, 2.5, 3.75]
)
problem_vrm.pair(survey_vrm)

# Predict VRM response
fields_vrm = survey_vrm.dpred(xi_true)

# Add an artificial TEM response. An analytic solution for the response near
# the surface of a conductive half-space (Nabighian, 1979) is scaled at each
# location to provide lateral variability in the TEM response.
n_times = len(times)
n_loc = loc.shape[0]

sig = 1e-1
mu0 = 4*np.pi*1e-7
fields_tem = -sig**1.5*mu0**2.5*times**-2.5/(20*np.pi**1.5)
fields_tem = np.kron(np.ones(n_loc), fields_tem)
c = (
   np.exp(-(loc[:, 0]-10)**2/(25**2))*np.exp(-(loc[:, 1]-20)**2/(35**2)) +
   np.exp(-(loc[:, 0]+20)**2/(20**2))*np.exp(-(loc[:, 1]+20)**2/(40**2)) +
   1.5*np.exp(-(loc[:, 0]-25)**2/(10**2))*np.exp(-(loc[:, 1]+25)**2/(10**2)) +
   0.25
)

c = np.kron(c, np.ones(n_times))
fields_tem = c*fields_tem

fields_tot = fields_tem + fields_vrm
fields_tot = fields_tot + 0.05*np.abs(fields_tot)*np.random.normal(size=fields_tot.shape)

Out:

CREATING T MATRIX
CREATING A MATRIX

Inverse Problem

Here, we invert late-time data to recover an equivalent source model. To recover the equivalent source model, only cells at the surface are set as active in the inversion.

# Define problem
survey_inv = VRM.Survey(srcListVRM)
actCells = (mesh.gridCC[:, 2] < 0.) & (mesh.gridCC[:, 2] > -2.)
problem_inv = VRM.Problem_Linear(
    mesh, indActive=actCells, ref_factor=3, ref_radius=[1.25, 2.5, 3.75]
)
problem_inv.pair(survey_inv)
survey_inv.set_active_interval(1e-3, 1e-2)
survey_inv.dobs = fields_tot[survey_inv.t_active]
survey_inv.std = 0.05*np.abs(fields_tot[survey_inv.t_active])
survey_inv.eps = 1e-11

# Setup and run inversion
dmis = DataMisfit.l2_DataMisfit(survey_inv)
w = mkvc((np.sum(np.array(problem_inv.A)**2, axis=0)))**0.5
w = w/np.max(w)
reg = Regularization.Simple(mesh=mesh, indActive=actCells, alpha_s=0.25,  cell_weights=w)
opt = Optimization.ProjectedGNCG(maxIter=20, lower=0., upper=1e-2, maxIterLS=20, tolCG=1e-4)
invProb = InvProblem.BaseInvProblem(dmis, reg, opt)
directives = [
    Directives.BetaSchedule(coolingFactor=2, coolingRate=1),
    Directives.TargetMisfit()
]
inv = Inversion.BaseInversion(invProb, directiveList=directives)

xi_0 = 1e-3*np.ones(actCells.sum())
xi_rec = inv.run(xi_0)

# Predict VRM response at all times for recovered model
survey_inv.set_active_interval(0., 1.)
fields_pre = survey_inv.dpred(xi_rec)

Out:

CREATING A MATRIX
SimPEG.InvProblem will set Regularization.mref to m0.

    SimPEG.InvProblem is setting bfgsH0 to the inverse of the eval2Deriv.
    ***Done using same Solver and solverOpts as the problem***
model has any nan: 0
=============================== Projected GNCG ===============================
  #     beta     phi_d     phi_m       f      |proj(x-g)-x|  LS    Comment
-----------------------------------------------------------------------------
x0 has any nan: 0
CREATING T MATRIX
   0  1.00e+00  4.02e+05  0.00e+00  4.02e+05    1.02e-01      0
------------------------- STOP! -------------------------
1 : |fc-fOld| = 0.0000e+00 <= tolF*(1+|f0|) = 4.0247e+04
1 : |xc-x_last| = 2.4394e-02 <= tolX*(1+|x0|) = 1.0450e-01
1 : |proj(x-g)-x|    = 9.5166e-02 <= tolG          = 1.0000e-01
0 : |proj(x-g)-x|    = 9.5166e-02 <= 1e3*eps       = 1.0000e-02
0 : maxIter   =      20    <= iter          =      1
------------------------- DONE! -------------------------

Plotting

fields_tot = np.reshape(fields_tot, (n_loc, n_times))
fields_vrm = np.reshape(fields_vrm, (n_loc, n_times))
fields_tem = np.reshape(fields_tem, (n_loc, n_times))
fields_pre = np.reshape(fields_pre, (n_loc, n_times))


Fig = plt.figure(figsize=(10, 10))
font_size = 12

# Plot models
invMap = Maps.InjectActiveCells(mesh, actCells, 0.)  # Maps to mesh
topoMap = Maps.InjectActiveCells(mesh, topoCells, 0.)
max_val = np.max(np.r_[xi_true, xi_rec])
ax1 = 3*[None]
cplot1 = 2*[None]
xi_mod = [xi_true, xi_rec]
map_mod = [topoMap, invMap]
titlestr1 = [
    "True Model (z = 0 m)",
    "Equivalent Source Model"
]

for qq in range(0, 2):
    ax1[qq] = Fig.add_axes([0.15+0.35*qq, 0.7, 0.25, 0.25])
    cplot1[qq] = mesh.plotSlice(
        map_mod[qq]*xi_mod[qq], ind=int((ncz+2*npad)/2-1),
        ax=ax1[qq], grid=True, pcolorOpts={'cmap': 'gist_heat_r'})
    cplot1[qq][0].set_clim((0., max_val))
    ax1[qq].set_xlabel('X [m]', fontsize=font_size)
    ax1[qq].set_ylabel('Y [m]', fontsize=font_size, labelpad=-5)
    ax1[qq].tick_params(labelsize=font_size-2)
    ax1[qq].set_title(titlestr1[qq], fontsize=font_size+2)

ax1[2] = Fig.add_axes([0.78, 0.7, 0.01, 0.25])
norm = mpl.colors.Normalize(vmin=0., vmax=max_val)
cbar14 = mpl.colorbar.ColorbarBase(
    ax1[2], cmap='gist_heat_r', norm=norm, orientation='vertical'
)
cbar14.set_label(
    '$\Delta \chi /$ln$(\lambda_2 / \lambda_1 )$ [SI]',
    rotation=270, labelpad=15, size=font_size
)

# Plot decays
N = x.shape[0]
ax2 = 2*[None]
for qq in range(0, 2):
    ax2[qq] = Fig.add_axes([0.1+0.45*qq, 0.36, 0.35, 0.26])
    k = int((N**2-1)/2 - 3*N*(-1)**qq)
    di_tot = mkvc(np.abs(fields_tot[k, :]))
    di_pre = mkvc(np.abs(fields_vrm[k, :]))
    di_tem = mkvc(np.abs(fields_tem[k, :]))
    ax2[qq].loglog(times, di_tot, 'k.-')
    ax2[qq].loglog(times, di_tem, 'r.-')
    ax2[qq].loglog(times, di_pre, 'b.-')
    ax2[qq].loglog(times, np.abs(di_tot-di_pre), 'g.-')
    ax2[qq].set_xlabel('t [s]', fontsize=font_size, labelpad=-10)
    if qq == 0:
        ax2[qq].set_ylabel('|dBz/dt| [T/s]', fontsize=font_size)
    else:
        ax2[qq].axes.get_yaxis().set_visible(False)
    ax2[qq].tick_params(labelsize=font_size-2)
    ax2[qq].set_xbound(np.min(times), np.max(times))
    ax2[qq].set_ybound(1.2*np.max(di_tot), 1e-5*np.max(di_tot))
    titlestr2 = (
        "Decay at X = " + '{:.2f}'.format(loc[k, 0]) +
        " m and Y = " + '{:.2f}'.format(loc[k, 1]) + " m"
    )
    ax2[qq].set_title(titlestr2, fontsize=font_size+2)
    if qq == 0:
        ax2[qq].text(1.2e-5, 54*np.max(di_tot)/1e5, "Observed", fontsize=font_size, color='k')
        ax2[qq].text(1.2e-5, 18*np.max(di_tot)/1e5, "True TEM", fontsize=font_size, color='r')
        ax2[qq].text(1.2e-5, 6*np.max(di_tot)/1e5, "Predicted VRM", fontsize=font_size, color='b')
        ax2[qq].text(1.2e-5, 2*np.max(di_tot)/1e5, "Recovered TEM", fontsize=font_size, color='g')

# Plot anomalies
d = [
    np.reshape(np.abs(fields_tot[:, 10]), (N, N)),
    np.reshape(np.abs(fields_tem[:, 10]), (N, N)),
    np.reshape(np.abs(fields_tot[:, 10]-fields_pre[:, 10]), (N, N))
]

min_val = np.min(np.r_[d[0], d[1], d[2]])
max_val = np.max(np.r_[d[0], d[1], d[2]])

ax3 = 4*[None]
cplot3 = 3*[None]
title_str = [
    "Observed at t=",
    "True TEM at t=",
    "Recov. TEM at t="
]

for qq in range(0, 3):
    ax3[qq] = Fig.add_axes([0.07+0.28*qq, 0.05, 0.24, 0.24])
    cplot3[qq] = ax3[qq].contourf(x, y, d[qq].T, 40, cmap='magma_r')
    ax3[qq].set_xticks(np.linspace(-30, 30, 7))
    ax3[qq].set_xlabel('X [m]', fontsize=font_size)
    if qq == 0:
        ax3[qq].scatter(x, y, color=(0, 0, 0), s=4)
        ax3[qq].set_ylabel('Y [m]', fontsize=font_size, labelpad=-12)
    else:
        ax3[qq].axes.get_yaxis().set_visible(False)
    ax3[qq].tick_params(labelsize=font_size-2)
    ax3[qq].set_xbound(np.min(x), np.max(x))
    ax3[qq].set_ybound(np.min(y), np.max(y))
    titlestr3 = title_str[qq] + '{:.1e}'.format(times[10]) + " s"
    ax3[qq].set_title(titlestr3, fontsize=font_size+2)

ax3[3] = Fig.add_axes([0.88, 0.05, 0.01, 0.24])
norm = mpl.colors.Normalize(vmin=min_val, vmax=max_val)
cbar34 = mpl.colorbar.ColorbarBase(
    ax3[3], cmap='magma_r', norm=norm, orientation='vertical', format='%.1e'
)
cbar34.set_label('dBz/dt [T/s]', rotation=270, size=font_size, labelpad=15)
../../../_images/sphx_glr_plot_vrm_inv_001.png

Total running time of the script: ( 0 minutes 19.337 seconds)

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